In this photo taken on Saturday, April 30, 2016, visitors look at steaming fumarolas at the Solfatara crater bed, in the Phlegraean Fields near Naples, Italy. Fields -- Campi Flegrei in Italian -- are a sprawling constellation of ancient volcanic centers.

MILAN --Three family members visiting a steamy volcanic field near Naples died Tuesday after an 11-year-old boy entered an off-limits area and his parents followed to try to rescue him, police said.

Police said the boy apparently slipped after he breached a fence surrounding the prohibited area at the Solfatara Crater in Pozzuoli. A 7-year-old boy who was a member of the same Italian family was nearby but remained outside the fenced-off area.

It wasn't immediately clear if the 11-year-old and his parents were overcome by gases or were killed as the result of an explosion of super-heated mud. Heavy rains in recent days may have played a role by creating more openings in the volcanic field's surface.

The crater is located in the Phlegraean Fields, a sprawling constellation of ancient volcanic craters frequented by Italian school children and tourists from around the world. The fields are scorching hot only a few inches below the surface.

Signs around the crater in multiple languages warn of the danger of burning from high soil temperatures and steam up to 320 degrees Fahrenheit. Visitors are told to stay clear of fumaroles, openings in the Earth's crust that emit steam and gases, and not to climb the slopes or breach the fences.

While the Phlegraean Fields are privately run, geologists monitor the area round the clock, checking temperatures and chemically analyzing gases. They have determined that the fields rose by about 12 inches over a decade.